1. Field of the Invention
The present invention relates to a printing device which carries out a stencil-making process in which an image is written on a stencil sheet by using a printing head and a stencil-attaching process in which the stencil sheet is wrapped around a drum, prior to a printing operation, and more specifically, relates to a stencil making and attaching method thereof.
2. Description of the Related Art
An explanation will be given of conventional stencil making and attaching methods in a printing device of this type.
As illustrated in FIG. 1, an elongated stencil sheet 100, is housed in an sheet housing section (not shown), and on the downstream side of the sheet conveying path, a thermal print head 101, which is a printing head for forming pores corresponding to an image in the stencil sheet 100, and a platen roller 102, which is pressed onto the thermal print head 101 with the stencil sheet 100 interposed in between, are installed.
On the downstream side of the platen roller 102 in the stencil sheet conveying path, a pair of first convey rollers 103 are placed in a virtually press-contact state, and the platen roller 102 and one of the first convey rollers 103 are designed so as to be rotated in synchronism with the platen roller 102 by the rotating force of a stencil motor, not shown.
On the downstream side of the pair of first convey rollers 103 in the stencil sheet conveying path, a pair of second convey rollers 104 are placed in a virtually press-contact state, and on the further downstream side of the pair of second convey rollers 104 in the stencil sheet conveying path, a pair of third convey rollers 105 are placed in a virtually press-contact state. The respective ones of the second convey rollers 104 and the third convey rollers 105 are designed to be rotated by the rotating force of a convey motor, not shown, in synchronism with each other.
Moreover, a sheet temporary stacker section 106 is installed at a lower position between the pair of first convey rollers 103 and the pair of second convey rollers 104. A stencil sheet cutter section 107 is installed between the pair of second convey rollers 104 and the pair of third convey rollers 105, and a stencil sheet set position detection sensor 108 is placed on the downstream side of the pair of third convey rollers 105 in the stencil sheet conveying path.
The stencil sheet set position detection sensor 108 is constituted by a reflection-type photo-sensor, and when the leading end of the stencil sheet 100 has been conveyed to a position below this, it detects the existence thereof.
Furthermore, on the downstream side of the stencil sheet set position detection sensor 108, a drum 109 is installed so as to be freely rotated, and this drum 109 is arranged to be rotated by the rotating force of a drum motor, not shown. A stencil sheet clamp section 110 is installed on the periphery of the drum 109, and the leading end of the stencil sheet 100 is clamped by this stencil sheet clamping section 110.
Next, in the above-mentioned arrangement, referring to FIGS. 1 through 5, an explanation will be given of stencil making and attaching methods of the stencil sheet 100. Here, suppose that the leading end of the stencil sheet 100 is only inserted between the thermal print head 101 and the platen roller 102 and that the drum 109 is positioned at a drum reference position at which the drum 109 is allowed to carry out the stencil sheet clamping process.
In this state, the stencil motor and the convey motor are both driven so that the platen roller 102, the first convey rollers 103, the second convey rollers 104 and the third convey rollers 105 are rotated so that the stencil sheet 100 is conveyed to the drum side. Here, when the leading edge of the stencil sheet 100 has reached the position below the stencil sheet set position detection sensor 108, the stencil sheet set position detection sensor 108 detects this, with the result that the conveyance of the stencil sheet 100 is stopped. In this case, the leading edge of the stencil sheet 100 is positioned at a stencil sheet setting position as shown in FIG. 1.
Next, as shown in FIG. 2, the stencil motor is driven to rotate the platen roller 102 and the first convey roller 103 so that, while the stencil sheet 100 is being conveyed, pores corresponding to a desired image are formed in the stencil sheet 100 through a heat-sensitive process by the thermal print head 101, thereby carrying out a stencil making process. The stencil sheet 100 which has been subjected to the stencil making process is temporarily housed in a temporary stacker section 106.
Next, as shown in FIG. 3, the convey motor is driven to rotate the second convey roller 104 and the third convey roller 105 so that the stencil sheet 100 is conveyed to the drum side by a predetermined amount. Then, this convey process by the predetermined amount allows the leading edge of the stencil sheet 100 to reach the position of the stencil sheet clamp section 110 of the drum 109, with the result that the stencil sheet clamp section 110 clamps the leading edge of the stencil sheet 100.
Successively, as shown in FIG. 4, the drum motor is driven to rotate the drum 109 in the arrow direction so that the stencil sheet 100 is gradually wrapped around the periphery of the drum 109. Further, as shown in FIG. 5, when this has been wrapped to a stencil sheet cut position, the rotation of the drum 109 is temporarily stopped so that the stencil sheet 100 is cut by the stencil sheet cutter section 107. After the cutting process, the drum motor is again driven so that the drum 109 is rotated so that the stencil sheet 100 is completely wrapped around the periphery of the drum 109, thereby completing the stencil attaching process. After completion of the stencil attaching process, a sheet of printing paper (not shown) is supplied to the drum 109 in synchronism with the rotation of the drum 109, and the sheet of printing paper is conveyed while being pressed onto the stencil sheet 100 wrapped around the drum 109; thus, during this conveying process, ink is transferred onto the sheet of printing paper through the pores of the stencil sheet 100, thereby carrying out a printing process.
In the above-mentioned stencil making and attaching method, as shown in FIG. 6, a position corresponding to a distance L1 from the leading edge of the stencil sheet 100 to the thermal print head 101 is defined as the image-formation start position, and as shown in FIG. 7(A), the leading edge of the stencil sheet 100 that has been subjected to the image formation is clamped at a clamp reference position of the drum 109 so that the image-formation start position is set at a fixed position. Then, a sheet of printing paper is conveyed in synchronism with the rotation of the drum 109 so that an image is always formed at a predetermined position on the sheet of printing paper.
However, the distance L1 from the leading edge of the stencil sheet 100 to the image-formation start position fails to provide an accurate length in the event of curing, etc. in the stencil sheet 100 as indicated by a hypothetical line in FIG. 7 or in the event of variations in the detection of the stencil sheet set position detection sensor 108. Moreover, with respect to the clamp position of the drum 109, clamping is not made at an accurate position when a slip, etc. of the stencil sheet 100 occurs with respect to the second convey roller 104 and the third convey roller 105, as shown in FIGS. 7(B) and 7(C). In this manner, when the image-formation position of the stencil sheet 100 and the clamp position with respect to the drum 109 are not accurate, the image-formation position of the stencil sheet 100 with respect to the drum 109 is offset from the regular position, with the result that an offset occurs in the print position on a sheet of printing paper, causing degradation in the positional precision in the printing process.